A simple circuit, a USB TV tuner, your
computer, and some powerful software
combine to make an amazing software defined
By George R, Steber
his unusual radio can be used to receive
conventional amplitude modulation (AM) and
frequency modulated (FM) stations, as well as
more specialized modes such as narrow band FM
(NFM), single-side band (SSG), continuous wave
(CW), and other signals. Its performance — while good — is
slightly below the best shortwave listening (SWl) receivers
or ham radios. However, we will show you how to improve
its performance substantially with tunable filters.
As it stands, it does a creditable job in many situations
and has features that many other radios do not — like
digital signal processing, spectrum analysis, and a waterfall
display. As an added bonus, you will be able to receive
frequencies from 24-1766 MHz in case you tire of the
classic shortwave frequencies.
In today's jargon, this receiver would be called a
software-defined radio (SDR) — a concept made popular
by the military. Typically, in an SDR receiver, an analog-to-
digital conversion (ADC) is made on the RF signal, and the
rest of the functions of a classic analog receiver are
performed on the digital signals in software. These
functions include tuning, filtering, and demodulation.
Using software to replace hardware allows more
versatility, provides more functions, and reduces hardware
For this design, we will use a simple RF converter and
a small inexpensive commercial digital TV module to
40 NUTS S VOLTS July 2015
provide the up-front hardware. The rest of the functions
will be done on the computer.
So, if you want to explore the world of shortwave
radio and learn more about this technology, read on and
definitely consider building your own software-defined
Software-Defined SW Receiver
Figure lisa block diagram of our shortwave SDR
receiver. Starting from the antenna, there is an RF
converter/mixer block. The purpose of this block is to
perform frequency up-conversion of the shortwave signals.
This is necessary because the next block requires RF
signals above 24 MHz. Therefore, a simple up-conversion
of the signals is performed using a mixer. Also in this block
are some analog filters to prevent strong AM and FM
signals from overloading the mixer and causing
The next block is a digital TV tuner — an amazing
device that does most of the work. It is connected to the
RF up-converter with a short coaxial cable to its antenna
terminal. In this device, analog signals from the up-
converter are tuned, changed to digital signals, and
conveyed via USB to the last block.
The final block is your home PC which performs all
the necessary digital signal processing. It should be
it was the simplest of times — it was the most complex of times. Indeed,
much like those words, this shortwave radio project is probably the simplest,
most complex one you have ever seen. It is simple because the main
hardware components are low cost, readily available, and easy to assemble.
It is complex because you need to download and install a special USB driver
and other software on your PC — and get it all working together. Once the
components are assembled and the software is installed, however, you will
have a very versatile shortwave radio that covers the 1-30 MHz range, and
provides many enjoyable hours of shortwave listening and experimentation.
Since the software is free and the components are cheap, you should be able
to build this receiver for around $25.
Mini DVB-T USB Tuner
The digital terrestrial TV age is in full bloom for
millions of viewers around the world — especially in
Europe, Analog terrestrial TV has been replaced by the
new Digital Video Broadcasting Terrestrial (DVB-T)
standard in many places. Viewers use mobile USB TV
tuners with their computers for reception of HDTV
broadcasts. Mobile tuners have compact dimensions —
about the size of a memory stick. They plug into and are
powered by the USB 2.0 port of your computer.
Figure 2 is a photo of a DVB-T tuner package
purchased on eBay for about $1 1 (including shipping). As
you can see, a remote control and an antenna are
included — ideal for watching TV on your laptop.
We will not be using the DVB-T tuner for TV viewing
in this project. Besides, it would not work in the US since
the DVB-T standard is not implemented here. Instead, it
will be repurposed to act as a wide-band tunable SDR
receiver. Hence, we will not need the other items in the
TV package. FIGURE 2. Low cost DVB-T package.
July 2015 NUTS! VOLTS 41
moderately fast, have
some USB ports, and a
sound card. The input data
to the computer is entirely
in digital form, coming
from the TV tuner via
USB. After applying digital
algorithms for filtering and
demodulation in the PC,
the processed digital data
is presented to the sound
card — all in the digital
domain. Finally, the sound
card is used to convert the digital data to audio to drive
speakers or headphones. Before we continue with details
of the design, let's take a closer look at the little tuner
device that makes this project possible.
CONVERTED SIGNAL DIGITAL SIGNAL
25-54 MHZ USB p0RT
FIGURE 1. Block diagram of SDR SW receiver.
Post comments on this article and find any associated files and/or downloads at
www.nutsvolts i com/index.php?/magazine/article/iuIy2015 i= Steber.
FIGURE 3. RTL tuner (with R820T and Realtek
RTL2832U inside) and BNC adapter cable.
Figure 3 is a photo of the DVB-T stick we are using
and a short interface cable that was bought separately.
Most DVB-T sticks have two main chips inside; a digitally
controlled tuner and an ADC that samples the baseband
signal and outputs the samples to a host computer
through a USB port.
A very specific DVB-T stick is used in this project —
one that has a Rafael Micro R820T tuner 1C and a Realtek
RTL2832U inside. These so-called RTL sticks are quite
common and available from many sources. Beware,
though, as some sticks have an E4000 tuner or some
other kind of tuner IC inside. Those will not work in this
project. Make sure your stick has an R820T inside.
The R820T tuner is crucial because it has the lowest
tuning range — tunable down to 24 MHz. Other tuners do
not have that low-end range. As we will see later on, this
capability makes our shortwave up-converter easy to build.
What makes the RTL stick so valuable is its inherent
ability to demodulate FM signals and transfer the
amplitude and phase information as raw in-phase and
quadrature phase (l/Q) samples to the computer via USB.
Annti Palosaari — a Finnish engineering student and Linux
developer — discovered this special radio mode.
Amazingly, this mode enables the tuner to output a
stream of eight-bit l/Q samples at rates up to two million
samples per second.
Once this discovery was grasped, enthusiasm grew for
the development of a cheap SDR. The group from Open
Source Mobile Communication (Osmocom) — particularly
Steve Markgraf — developed a basic set of drivers and
utilities to communicate with the RTL dongle, After that,
other software developers began writing code to use
these drivers and provide user interfaces. One of those
programs, SDR# (pronounced SDR Sharp} is probably the
most popular one. We ll discuss it in more detail later on.
RF Converter and Assembly
FIGURE 4. Detailed schematic — shortwave radio
The front end of our
shortwave receiver is a
An up-converter is a
circuit that adds a
constant frequency to
the received frequency —
the one received at the
antenna. An RF mixer
and local oscillator can
be used to make a
simple up-converter. If
we use a local oscillator
frequency of 24 MHz,
then the output of the
mixer will contain the
received signal plus 24
MHz. Both the local
oscillator and the mixer
functions can be handled
by a general-purpose
If you search the
Internet, you will see
some designs that use
42 NUTS S VOLTS July 2015
down-converters with 125 MHz local
oscillators. These circuits are overly
complicated and harder lo build. They are also
more prone to spurious signals and more
costly. Our little up-converter costs quite a bit
less and performs better in most instances.
The design of the frequency converter is
straightforward. However, there is one slight
wrinkle as two different front-end filters are
presented: broadband and tunable. This gives
diverse users the option to utilize the SDR for
different applications. The broadband fitter is
low cost and works well over the entire 1,5-30
MHz range, but has more noise due to inter-
modulation from strong in-band stations. This
option works well for strong AM SW stations
from around the world without the bother of
peaking a filter.
On the other hand, the tunable filter has a
narrower band and requires manual signal
peaking, but has less noise and can dig out the
weak stations. This option is better for receiving
amateur radio and other low power stations.
Figure 4 is the circuit for the 24 MHz up-
converter. A parts list for the project is shown
in Figure 5. Most parts for the receiver are
readily available from sources like Mouser, Digi-
Key, and Jameco. The SAG 12 is the only chip in
the circuit, and is used for mixing and local
oscillator functions. The up-converter frequency
is determined by crystal CR1 , It needs to be 24
MHz or slightly higher. The exact frequency is
not critical, as an offset frequency will be used in
the final software to compensate. So, any frequency in the
range of 24 MHz-24,6 MHz will work fine. It is important
that the capacitors C7 and C8 be good quality — like
Class I NPO ceramic or even silvered mica — to minimize
temperature drift. C7 is probably not needed. Leave it out
if the circuit oscillates without it.
The circuit is powered from eight to 12 volts DC and
is regulated via the 78L05. Use a linear voltage supply as
opposed to a switching type to avoid noise. Batteries work
well too, as the current draw is small.
As mentioned earlier, there are two RF input filter
choices. Choosing the upper jumpers on jPI and JP2
connects the bandpass filter (BP). You don't actually need
to build both filters — only build the one you want. The BP
filter is designed to remove strong stations from the AM
broadcast band and FM band that would otherwise
overload the mixer.
The other choice is the tuned filter. It requires hand
wound high Q coils and a variable capacitor to peak the
signal. (See the Parts List for details.) For this filter, only
specific bands are covered. So, with this option, if you
want to cover the whole HF range, you will need to
Class 1 NPO ceramic- see text
Class 1 NPO ceramic
1/4W 5% resistor
1/4W 5% resistor
1/4W 5% resistor
1/4W 5% resistor
Fastran RF inductor, Mouser
Fastran RF inductor, Mouser
Fastran RF inductor, Mouser
Fastran RF inductor, Mouser
1ST, T50-6 core
9T, T50-G core
28T r T68-6 core
13T, T5Q-6 core
56T, T68-6 core
2GT, T50-6 core
5V regulator, Jameco
Jack 3.5 mm
RF mixer/osc, Mouser
24 MHz - see text
Figure 5. Parts List for SDR SW receiver.
provide switching between coils. The improved
performance may be worth it.
The variable capacitor CV needs to have a wide range
— typically 15 pF to 400 pF. These capacitors are getting
more expensive and harder to find, but can still be found
on the Internet for around $15.
The output "up-converted" signal is provided to )2
(BNC) through a tuned output stage designed to match
the 75 ohm input of the RTL antenna. Figure 6 is a
breadboard for the tuned filter receiver. In general, this
open layout is not recommended for RF circuits. For best
performance, the mixer circuit should be built on a PCB
(printed circuit board) and housed in a small metal box.
Although a PCB design is not available, it should be easy
to make — even for beginners,
A short coaxial cable is connected from |2 to the RTL
stick's antenna terminal. The RTL usually has an MCX type
connector as its antenna terminal. Because an MCX plug
is very small and difficult to solder, it is suggested that you
buy a ready-made adapter cable — an MCX plug with a
short RG31G cable to BNC. Figure 3 shows such an
adapter cable. To reduce computer noise, a USB
July 2015 NUTS1 VOLTS 43
FIGURE 6. Breadboard of tunable filter shortwave radio.
extension cable may be used to move the RTL farther
away from the computer.
First of all, do not use any of the software that came
with your RTL device. It was designed for a different
application. For our radio, the RTL stick requires a special
USB driver and a graphical radio interface. You will need
to install Zadig for bulk interface drivers and then SDR#
for the radio interface. Once Zadig is run, you will have a
new USB driver named "Bulk-In, Interface (Interface 0)/'
That is what the RTL device uses.
After you install SDR#, you will be able to select this
driver to use with your radio. Also note that — depending
on your PC operating system — you may need Microsoft's
Net 3.5 for the installation to work.
The software installation discussed earlier is too
detailed to present here. The following websites can take
you through the procedure. Perhaps the best site that
covers the entire installation is www.rtl-sdr.com/rtl-sdr-
quick-starf-guide. Other sites to check are
http://inst.eecs.berkeley + edu/"'ee123/fa12/rtLsdr.htmL
The installation may seem a bit daunting when you are
just starting out, but remember, once you get it working it
will be well worth the effort.
A screenshot of SDR# tuned to WWV is shown in
Figure 7. It's interesting to see atmospheric fading in the
44 NUTS \ VOLTS July 2015
waterfall plot. Running SDR# is like having
a radio lab at your disposal More details
on its operation can be found at
Now, let's take a look at some of the
screen buttons. In the upper left corner is
the Play button — which obviously starts
the program. What is not obvious is that
some settings cannot be changed while it is
running. So, if you cannot set something,
make sure the program is stopped. The first
thing you should do is select the USB driver
you installed by pressing the down arrow
located next to the Play button. Choose
RTL-SDR/USB from the available options.
Next, click Configure. Choose a sample
rate of 1.024 MSPS and Quadrature
sampling. Leave the other boxes blank.
Notice that there is a slider to select the RF
gain. Set it to about 14.4 dB. It can be
changed later if it's too low. There is
another box at the bottom that allows
correction for the crystal frequency inside
the RTL, We will discuss that later.
Now, look below the Play button.
There are numerous buttons for selecting NFM, AM, and
so forth. In this screen area is a box labeled Shift. Click
this box and enter: 24,000,000. Notice the 'minus' sign.
This number corresponds to the crystal frequency in the
up-converter and will provide the first rough correction —
it enables SDR# to read out the RF 5W frequency directly.
Now, move down to the Audio section. The
Samplerate and Input boxes should be grayed out. This is
because data is being sent via USB not audio. In the
Output box, select the sound card that you are using. It
will produce the sound you will hear.
Let all of the other settings in SDR# be at their default
for now. You will have lots of fun experimenting with them
later on. Connect a long wire antenna and ground
counterpoise. Make ihem as long as possible -25 feet or
more. With any luck, you should now have a functioning
Three automatic gain controls (ACC) are available.
The ones in the Configure box do not work well. The AGC
on the main screen works well on strong stations. For
weak stations, leaving AGC off and manually adjusting the
RF gain may work best.
Other SDR# settings are usually a matter of
preference, but here are some guidelines. Generally, a
moderate value of Zoom should be used unless you are
calibrating the radio. This makes it easier to click on a
peak to select a station. Use a small number for FFT —
typically 4096 to get good computer performance. Use a
bandwidth that is appropriate for the signal being
monitored: CW 800 Hz; SSB 2.8 kHz; and AM 10 kHz. A
SDR? vl.O.0. 1 1 35 ■ fQ imbalance; Gain = 1 ,000 Phaw = 0,000 -
Alt mSDS U 5 B
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FIGURE 7. SDR# opening screen — radio tuned to 10 MHz WWV.
filter order of 40 works well and will
have little effect on program speed.
Tuning in an 5SB station can be a
challenge as there is no carrier peak.
Tune to the left or right of the signal
depending if USB or LSB is used, and
adjust the frequency dial slowly until
voice is intelligible.
You may want to calibrate the
frequency dial more accurately. This is
a two-step procedure as there are two
crystal oscillators used here: one in the
RTL and one in the RF converter. To
calibrate the RTL, uncheck Shift and
connect a short wire of three or four
feet directly to an RTL antenna
terminal. This should allow you to pick
up a NOAA weather station in your
area — usually around 162.400 MHz.
Look up exact frequencies on the
Internet. Note that NOAA uses NFM,
Click Configure and adjust the
Frequency Correction (PPM) until the
dial reading corresponds to the NOAA
To calibrate your HF SDR receiver,
check Shift and tune to station WWV using USB — not
AM. WWV at 10 MHz is useful for calibrating. Use a high
value of FFT resolution — around 131,073 — so you can
see WWV peak dearly in the spectrum. Adjust the Shift
value until the dial reads the peak frequency accurately to
within 50 Hz. tt will drift periodically, but that is the nature
of uncompensated crystal oscillators.
As noted above, if you bypass the HF converter and
use an antenna directly connected to the RTL, you can
take advantage of the very large frequency range of this
device — up to 1,766 MHz. Thus, you may be able to
directly pick up FM stations (and other stations like
NOAA) and two-meter ham repeaters if they are close
Hopefully, by now you should be having fun with your
SDR SW receiver. If you are having difficulties, go to
http://sdrsharp.corn or the Yahoo SDR group and they
may be able to help.
Shortwave radio is more exciting now than ever
before. Many new AM broadcasters from China, Cuba,
Europe, and other places will keep you informed and
entertained. Religious broadcasters also abound.
Unfortunately, many of the interesting stations only appear
at night due to propagation conditions. Fortunately, the
powerful ones come booming in even in the daytime.
Listening to ham radio operators using SSB and Morse
code is also fun — especially during contests. Other
stations are using SSB too, such as Maritime WX on 8.763
MHz and Aviation WX on 10.051 MHz. These generally
use synthetic voices. If you are lucky — as I was — you
may hear one of the mysterious 'numbers' stations using
USB around 13.199 MHz, However, they are seldom in
the same spot twice. Also interesting are utility stations
such as WLO, which transmits on many frequencies and
provides high seas communication and weather
One of the exciting things you can do with this radio
is receive data transmissions such as WX FAX, S1TOR,
RTTY, BPSK, WSPR, and Easy Pal S5TV. These modes can
easily be decoded with available software, but to discuss
this further would require another magazine article!
If you like playing around with this radio and find the
subject fascinating, you may want to consider becoming a
radio amateur. Hams are involved with building and
studying receivers, transmitters, antennas, satellites, EME,
microwaves, and experimenting with new radio modes
such as WSPR, BPSK, Packet Radio, and more. If you are
considering joining the fraternity of radio amateurs, the
ARRL website may be the place to start. Be sure to check
out the new column here in Nuts & Volts , as well.
In the meantime, have fun with your new SW radio.
July 2015 NUTS S VO ITS 45